Garden center. Or farm supply. I'm not a dyer, so I'm not sure that's the FORM of urea they're looking for, but they sure as h*ll sell it. (It's a commonly used fertilizer, though it's not as common now as it used to be)

Aw, we've been dying with urine for thousands of years. It was the only way to get black dye way back in the way back.

A good friend of mine has a sister who is a master weaver. Said master weaver decided to start growing a fantastic dye garden and dye her own fibers using ancient techniques, but she wasn't getting the right results. It turns out that her urine wasn't potent enough because she's a vegetarian. It was pretty funny when she started having to ask meat eating, beer drinking friends to help her in a new way with her dye projects.